Solstice
A solstice is one of the two days each year, around June 21 and December 21, when the Sun reaches its highest or lowest noon position in the sky.
Definition
A solstice is a day when the Sun is at its highest or lowest point in the noon sky for the year: the June solstice, around June 21, is the longest day of the year in North Carolina and the start of summer, and the December solstice, around December 21, is the shortest day and the start of winter. A solstice occurs when Earth's axis is tilted most directly toward or away from the Sun, so the Sun reaches its greatest declination north or south; around June 21 it is directly overhead at the Tropic of Cancer, $23.5^\circ$ N, giving the Northern Hemisphere its longest day and the Southern its shortest, and around December 21 it is overhead at the Tropic of Capricorn, $23.5^\circ$ S, with the situation reversed. Precisely, the solstices are the two instants when the Sun's ecliptic longitude is $90^\circ$ or $270^\circ$, equivalently when its declination reaches $+23.5^\circ$ or $-23.5^\circ$. At latitude $\phi$ the noon solar altitude at the solstices is $90^\circ - \phi \pm 23.5^\circ$, and day length is maximal or minimal. Because Earth's orbit is elliptical, the intervals between solstices and equinoxes are unequal, and the earliest sunset and latest sunrise in the Northern Hemisphere fall on different dates from the December solstice itself due to the equation of time.
Example
On the June solstice, Charlotte has about $14$ and a half hours of daylight, and on the December solstice it has under $10$ hours. On the June solstice the Sun rises well north of east and sets well north of west, and inside the Arctic Circle it never sets, while on the same day at the South Pole the Sun does not rise at all. In Charlotte the earliest sunset occurs in early December and the latest sunrise in early January, even though the December solstice around the 21st has the shortest total daylight. The offset arises because solar noon shifts later by about $30$ seconds per day in December.
Key Insight
The longest day is not usually the hottest day, because the land and oceans keep warming for weeks after the solstice, so the hottest weather comes in July and August. The word solstice means "sun stands still," and for a few days around each solstice the Sun's rising and setting points stop moving along the horizon before reversing direction. Precession slowly moves the solstice points along the ecliptic, and the solstice dates shift within the calendar by the leap-year cycle. Monuments such as Stonehenge and Newgrange were aligned to solstice sunrise or sunset, showing that the solstices were among the first astronomical events humans tracked.